If you're experiencing problems with OneDrive syncing after updating to the latest Windows Technical Preview build, I might have received a fix for you. Microsoft contacted me after I started complaining at this problem on Twitter, and it seems like the fix they provided will work.
To recap what's happening here, Microsoft issued a new build to Windows Technical Preview users yesterday afternoon. While most of the changes in this build are fairly innocuous, there's one biggie: OneDrive no longer uses the smart files system from Windows 8.x—which was wonderful—apparently because it confused too many people. (And they would go offline and not have access to files they assumed were on their PC.) Based on the level of complaining, I suspect that Microsoft will do the right thing and allow power users to re-enable smart files. But that assumes that there isn't in fact an underlying technical issue that made smart files untenable. We'll see.
(Microsoft also revealed that it was removing the OneDrive modern app from Windows 10, and some other changes. But the removal of smart files is of course the big news.)
In the meantime, a number of users who upgraded from the previous Windows Technical Preview build to the new build, build 9879, are experiencing sync issues. There are a couple of major examples of this, but what I saw was a red bang on the OneDrive tray icon and the message "Your OneDrive won't fit on this PC."
Regardless of whether that was true—Windows supports selective sync anyway, so why would it matter?—it turns out the error message is a bug, and in my case it's related to the sheer volume of content I'm syncing between the PC and the cloud. (You may recall that I've been experimenting with, and writing about, ways to get content from my PCs into OneDrive's now-unlimited storage.)
So that will be fixed, Microsoft tells me. In the meantime, here's the workaround:
1. Stop the skydrive.exe process.
2. Move all of the files from the OneDrive folder to a new location, preferably on another drive or PC just in case any aren't synced.
3. Delete the local OneDrive folder. (Mine is in the root of the D:\ drive, but it's more typically found in C:\Users\Paul, where "Paul" is your user name.)
4. Start skydrive.exe again.
When you complete this, you'll see a window I couldn't get to previously, where you can configure how OneDrive syncs, either all of it or certain folders. Based on the volume of content I have in there, I had to tediously step through the latter option.
Once you complete this process, you'll be notified that Windows has created a new OneDrive folder and it will sync. Mine is still syncing as I write this. But it is syncing. (And will take quite a while to complete.)
Let me know if this does—or doesn't—work for you. No, it doesn't fix the central issue of smart files being removed. But at least you should be able to get up and running again. This is pre-release software after all, and hopefully you haven't foolishly trusted important data to it. (As I have.)